1. Field of the Invention
This invention relates to a Schottky barrier type compound semiconductor device and more particularly to a Schottky barrier type compound semiconductor device provided with an improved insulation layer.
2. Description of the Prior Art
Particularly known among Schottky barrier type compound semiconductor devices is the type in which a layer of, for example, titanium, platinum, molybdenum, or tungsten is mounted on a substrate prepared from gallium arsenide. Also known in recent years is a compound semiconductor device in which the Schottky barrier is formed by, for example, a metal such as niobium, tantalum, or vanadium, as set forth in the Japanese Patent Application Disclosures 36,878 (1976) and 60,568 (1976).
Most of the compound semiconductor devices of the above-mentioned type are constructed by forming an insulation layer on an N type gallium arsenide substrate, making a hole in the insulation layer, mounting a Schottky barrier-forming metal layer on the substrate through the hole and further depositing on said Schottky barrier metal layer another metal layer used to bond the compound semiconductor device to an external element. Already proposed are Schottky barrier type compound semiconductor devices whose insulation layer is formed of silicon dioxide (refer to "GaAs Schottky Barrier Diodes ECL-134" by Yasuo Sato et al appearing in "Electrical Communication Laboratories Technical Journal", Vol. 18, No. 7 (1969), pp. 1,717 and "High Performance Ni-Pd/GaAs Schottky Barrier Diodes" by A. Nara et al disclosed in "Digests of Technical Papers in the 9th Conference on Solid State Device, Tokyo", C-2-4 (1977), pp. 55). However, the proposed Schottky barrier type compound semiconductor devices have the drawback that the above-mentioned insulation layer of silicon dioxide has a thermal expansion coefficient widely different from that of the substrate crystal, giving rise to cracks in said insulation layer or its peeling from the substrate during heat treatment. Therefore, another Schottky barrier type compound semiconductor device has been proposed in which the insulation layer is formed of phosphosilicate glass (PSG) prepared by doping a silicon dioxide layer with phosphorus at a low concentration of, for example, ca.3.times.10.sup.20 /cm.sup.3.
Even the last mentioned Schottky barrier type compound semiconductor device still has the drawbacks, as experimentally disclosed by the present inventors, that (1) the PSG layer tends to peel from the substrate during the thermal deposition of a Schottky barrier-forming metal layer or during the subsequent heat treatment; (2) the bonding of the semiconductor device to an external element increases the capacitance of the Schottky barrier; (3) break-down voltage falls; and (4) undesirable increases arise in the n value denoting the forward voltage-current property, thus lowering the yield of the product.